High frequency amplifier



Jan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER Filed Jan. 27, 1951JE; EEE. I

,HIFIII lll. Il Il I .Lrl.i|\|1| |l /lVl/E/VTO By J. A. MOE TON ATTORNEYJan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER 4 Sheets-Sheet 2Filed Jan. 27, 1951 /VVE/VTOR J. MORTO/V BV #mw ATTORNEY Jan. 29, 1957J. A. MoRToN 2,779,891

HIGH FREQUENCY AMPLIFIER.

Filed Jan. 27, 1951 4 Sheets-Sheet 3 FIGZB /Nl/E/VTOR J. A. MORTO/v ATTORN-V Jan. 29, 1957 J. A. MORTON HIGH FREQUENCY AMPLIFIER 4Sheets-Sheet 4 Filed Jan. 27, 1951 FROM LOCAL OSC.

OUTPUT SIGNAL SOURCE /NPUT CAV/TY l L/05 s/GNA L SOURCE /Nl/EN To@ l A.MORTO/V FROM LOCAL OSC ATTO/@NE ,V

United States Patent O HIGH FREQUENCY AMPLIFIER Jack A. Morton, NeshanicStation, N. J., assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application January 27, 1951,Serial No. 208,203

12 Claims. (Cl. 315--3.5)

tric eld of awhigh frequency wave transmitted along the path causing thehigh frequency wave 'to be amplified.

lt is an `object of this invention to increase the signalto-n'oiseYratio of such a device.

'it Ais also an object of the invention to launch an electron stream ina traveling-wave type tube with a very small transit angle between thegun rcathode and the helix.

Another object of the invention is toincre'ase the gain ot such anamplifier.

it is a further object of this invention to `maintain broad band widths`in such an amplier while decreasing the noise generated therein.

Another object 'of this invention is to ,prevent formation of noiseproducing 'plasma within such a device by eliminating the potentialpockets along the electron beam.

Another object of the invention is to combine features of microwavetriodes with thoseof traveling wave tubes so that beneits ofboth'devices may be realized in a single unit.

in devices of the type described above which are now known as travelingwave tubes, it is well `known that the phase velocity of the ,highfrequency wave must be approximately equal to the average velocity ofthe electron stream in y'order that there will be `an appreciable energytransfer. The phase velocity `of a high 4frequency wave in airwill be oftlreorder of vthe vspeed of light while the velocityfot the'electronsemitted by an `electron gun having a beam voltage on the orderof 1500 volts will be about one-thirteenth `the speedof light.

In traveling wave tubes `of the type described in a copendingapplication Serial No. 704,858, tiled October 22, 1946, by l. R. Pierce`(United States Patent No. 2,602,148y issuedluly l, 1952), synchronousvelocities are approximated by causing the high frequency or travelingrwaves to follow a helical transmission path of the proper pitch whilethe trajectory of the electron stream is a path adjacent and 'parallelto the longitudinal axis of the helical4 path. A series lofi`cylindrical support rods of any suitable insulator material which willnot introduce excessive loss or capacitance in the `tube aroused toposition the helix in the glass envelope in order to facilitateprojection of the electron stream along the length of the coil withoutgreat loss of electrons to the coil conductor.

It has been found that traveling wave tubes tend to have relatively lowsignal-to-noise ratio due to sources of noise within the tube. lnattempting to determine what these sources are ithas been discoveredthat in a tube such as described `in the Pierce application the transitangle between the electron emitter and the helix and the randomeaptureof current fromthe beam rissuch that the `electron `beam enters-the helix fasalm'ostfpure shot vnoise before `it receives'thesignalimodulation.

Another shurceof noise has Vbeen A'discovered' to `be ion oscillationsin plasma which form within the tube, `particularly `in the region ofdeep potential pockets,

A further source of noise has been traced by applicant to Ythevariablepressure and hence variable contact resis'tance and capacitancewith which the loss coated ceramic rods of `a tube as disclosed in thePierce application are held. in contact with the helix. Techniques aredisclosed in my copenlding application Serial No. 208,204; tiled lanuary27,V 1951,- for the construction of a unitary helix assembly whereby the4ceramic rods are held in rigid contact 'with thehelix by glaze appliedthereto.` Loss material is applied about the circumference of a 'centerpor-- tion `of the entire assembly. Since the glaze `insulates 'the rodsfrom the helix, the effect of Contact resistance is substantiallyeliminated; capacitance between the wires and support rods is also madeuniform. A higher loss per unit length over which loss material isapplied is 'also obtained by using these techniques so that theloss-free interaction recien is increased, resulting in a higher igainfor the same total length of helix. t

in traveling wave tubes it has been necessary to make specialprovisioinfor preventing self-sustaining oscillations which .may result from wavesreliected from the output section due tio an imperfect impedance matchof the helix to the output section. One method as vdisclosed in myaforementioned copending application applies aV coating othigh lossconductive material such as aquadag over the center portion of the helixassembly. In addition to .attenuating reiiected waves this coating ofloss material also promotes `isolati-on of the input and output sectionsci the amplifier and therefore tends to `limit disturbances `due toreflection therebetween.

Fluctuations in an `electronstream as it leaves the electronemissivesurface are largely velocity fluctuations but as is known, thesefluctuations become density or convection current iiuctuations at someiinite transit angle from the cathode` andrat ysome point they approachpure shot noise. Applicant has utilized this concept to achievea `highsignal-to-noise ratio by launching an electron beam down a helix with asmall transit angle between gun cathrodenandlhelix vso thatthe noiseeitect of the beam `issubstantially below the level of pure shot noise.Intone emhodiment, described below, the signal is impressed fon thestream in the region fpreceding the helix where the noise is greatlyreduced by space charge suppression and asrnall transit angle. ThisVregion is` just the cathode-grid input vof a microwave triode having ahigh transconductance so that both signal and noise are amplified bythegain in the regionpreceding the helixbefore noise due to transit timeo1- random capture `eect enterthe rstrearn. The helix then simply addsmore gain and the original signal-to-noise ratio of the space chargeregion is preserved if the gain precedingthehelix is adequate.

A structure in accordance with the invention whereby anwelectron `streammay belaunched with asmall transit angle with `provisions for impressinga signaLto be ampliiied uponthe stream before itenters the helix isobtained by combining a microwave triode input section, i. e., thecathode and grid sections, with a traveling Wave tube. The helix:replaces the anode of the triodeso that the transit angle `oftheelectron stream from the triode cathode as it enters the helix islextremely small, particularly in comparison with conventional traveling'wave tubes of the prior art. The transit angle is also small vrelativeto the 'transit angle required for velocity fluctuations from thecathode to reach` the level ofv pure shot noise convec `tion`current*iluctuations and in `fact locates the helix Vinput 'in theregion of or as close as possible tothe potential minimum just in lfrontof the cathode. This region resulting from space charge is well `known`and is .illustrated for example 1in fElectronics"by l. Millmanjand iS.Seely, McGrawHill, 1941, `at page 223. The input signal modulates thebeam convection current in the cathode-grid region and the outputamplified signal is taken from the far end of the helix as fromconventional traveling wave tubes. A collimating grid may be added tothe helix input to insure a parallel sided beam so as to reduce noisedue to plasma ion oscillation.

The combination of a triode and a traveling wave tube in accordance withthe invention is herein called a hybrid tube. One feature of a hybridtube is that the overall gain is greater than that of a conventionaltraveling wave tube of the same helix and beam dimensions due to thetransconduct-ance of the region preceding the helix in which the signalis impressed on the electron beam. Further, the bandwidth of the deviceis greater than that of a similar microwave triode since in the latter,the bandwidth is limited by the output section and in the hybrid tube,this section is replaced by the broad band helix.

`Other features and objects of the invention may be better understood byreference to the following detailed description when read in accordancewith the following drawings, in which:

Fig. 1 is a pictorial representation of a hybrid tube of the typedescribed herein in detail;

Fig. 2A is a cross section view of the device including an input and anoutput circuit and showing particularly the general relation of theelectrodes in the device;

Fig. 2B illustrates in diagram only the input section of a prior arttraveling-wave tube;

Fig. 2C illustrates as modification of the input section of Fig. 2A toshow the relation of the electrodes when the collimating grid is notincluded;

Fig. 3 is an enlarged cross section view of the input section of thedevice of Fig. 1, showing the detailed assembly when the collimatinggrid is included;

Figs. 4A and 4B compare, in a pictorial manner, electron beams with andwithout crossover;

Figs. 5A and 5B illustrate by diagram the collimating grid feature ofthe present invention;

Fig. 6 is an exploded perspective view of the collimating grid 95 ofFig. 3 and its immediately adjacent structure;

Fig. 7 is a pictorial view of the base of the device of Figs. l and 3;and

Figs. 8 and 9 illustrate, partly in diagram, features of the inventionas applied to devices employed as modulators. Y

Gain

In general waves may be excited on a helix by any one or combinations ofthree mechanisms. These mechanisms are the following:

(a) By injecting an electron stream whose convection current is signalmodulated;

(b) By injecting an electron stream whose velocity is modulated; and

(c) By applying an A. C. signal field to the beginning of the helix andinjecting an unmodulated electron stream.

In any of these situations when the electron stream velocity is insynchronism with the helix wave Various amounts of four different wavesare excited in the system comprised of the helix and electron stream.There is one backward traveling wave and three forward traveling waves.The three forward waves are as follows:

l. A growing wave traveling slightly slower than the beam direct currentvelocity;

2. A decaying wave traveling slightly slower than the beam velocity; and

3. A constant amplitude wave traveling slightly faster than the beamdirect current velocity.

In the ensuing discussion attention will be focused only on theincreasing wave, assuming the reflected backward wave is eliminated inactual practice by perfect output termination or by loss. The other twoforward waves will be neglected on the basis that if useful gain is tobe achieved, the tube will be suiciently long so that the growing wavewill be many times greater than the other two in the output region ofthe helix.

When all three methods of wave launching are used, the voltage amplitudesquared of the growing wave at u0=direct current beam velocity;

where:

K1 is the helix impedance;

lo is the direct current beam current; Vo is the direct current beamvoltage;

In the conventional traveling wave tube, in so far as signal modulationsare concerned, vs=qs'=0, and the signal is applied only as a voltage Vsto the beginning of the helix.

In such a case, the maximum available gain becomes Where TT is the gainof a conventional traveling wave tube, 'and N is the length of thelossless helix-electron stream interaction space in wavelengths.

This reduces, in decibel, to

IT(db)=9.54+47.3 CN

In one manner of operating the hybrid tube, when the transit `angles inthe spaces between grid and helix are very short, and when the helix isshorted at its input, then,

vs=l7s=0 and the maximum available gain becomes rH= (ymR/aKJ/a)(rl/CN)where IH is the gain of a hybrid tube; gm is the transconductance of thecathode-grid region;

and

and will be termed the beam impedance; or

/s i/a imdb) :10 10g,(g'" R"2-I-)9.54+ 47.3 CN

Hence:

The maximum available gain of the hybrid tube is thus greater than themaximum available gain of the conventional traveling wave tube by anamount which is a func tion of the transconductance, the beam impedanceand armeni T the helix impedance. .In a typical experimental travelingwave tube, the lhelix .had thel'ollowing parameters: Kleine ohms Vo:1600 volts 14T-'12X 1'0-3 amperes so that recount LEO-T Ohms Thus:

a infix 16W- 6i :2.98K 103 gm fl() gm For `a typical close spaced triode'of the type used for an inputsection 'of the hybrid tube,

gm=50,000 l0*6=5 X 162 (for Iu=30 ma.)

Therefore,

Bandwidth Whenever one transmission system with a given fieldconfiguration is `matched into another of different field configuration,local distortions of the field in the region of transition must be setup. These distortions can be resolved into other higher order fieldmodes than the principal modes of either of the two transmission systemsand as `such they 'are primarily local in nature and do not propagatewell into either transmission system. They do, however, represent storedenergy and as such can be represented as lumped capacitances shuntedacross the principal modes of the two adjoining systems.

Let us represent the effect of such wave transforming o discontinuitiesvas etective capacitances lumped across the various signicant electronstream interaction spaces.

Because the helix-to-wave-guide transformation may be made as gradualaswe please and since the helix impedance is itself low, we may usuallymake the bandwidth of a helixto-wave-gnide transformation much broaderthan can be achieved in the input gap vof a hybrid tube where we vhavethe sudden discontinuity of the cathode-grid gap capacitance andconductance.

It we define the band of an amplifier BMX) as that interval within whichthe voltage gain is Within a factor of times the maximum-gain, then:

defines BMX); where:

Fao is `the gain at the band center; and

Ilu is the gain `at a `frequency removed from the ban center.

Analogously, .if we `det`me-the band of a simple anti yresonant circuitas `Be('n then:

6 15%)) .denses .nieu

where:

Yc'(wu) is the admittance at the ibanlfcenter ofthe `circuit alone; andy Ycfw) is the admittance at a frequency removed from -the band center.

It follows, therefore, that where:

Gc is the circuit conductance; and Ce is the effective circuitcapacitance.

Since amplifier gain is inversely proportional to the product of theinput and output circuit admittances, ninazX. Two important cases are'to be here considered.

(a) The band is equally shaped by the input and output as in theconventional traveling wave vtube in which case input 'and output Qs areequal and l GT= and is the helix conductance; and,

CT is the discontinuity capacitanceassociated with the fieldtransformation from 'helix tow've guide.

Therefore and l G1 f Baco-2T CTvX t1 y where BT(A) is the bandwidth ofthe traveling vvave tube. For )5x2 at the 6 db down points:

(b) The band is shaped only by the. input; this is the case of thehybrid tube. Then, Qa Q1 and X=rt2 'so that where:

Gin is the input. conductance of the 'hybrid tube at the electronstream; and Cin=C11+Cp1 and is the total input capacitance -at the sainepoint Where: C11 is the gap capacitance; and, 1 l Cpi is the effectivecapacitance of the matching resonator; and BMA) is the bandwidth of thehybrid tube.

At the 6 ydecibel down points, X :2 so that 1 Grill! B A H( 2T 01H1/3 Ina speciic hybrid tube of the type described below, using a close spacedtriode input sectionyitivasnot possible at 4000 rnegacycles to tune onthelirst node since it exists inside the tube. The total effective inputcapacitance was thus very large and in an actual case lthe inputbandwidth was measured to be about 500 megacycles corresponding to aninput capacitance ofV about 58 micro microfarads for the case in whichG1H2gm=2 son00 l'oemhes By designing the input section of 'the hybridtubefso as to permit resonating at the rst node inside the envelope, thebandwidth may be raised tothe order of `1000 megacycles which is lalso atypical value for conventional traveling wave tube. l

In general, however, the straight traveling wave tube can achievesomething of the order of twice the bandwidth of the best hybrid tubewith about the same amount ofdesign effort. This sacrifice in bandwidthwill in many applications be warranted in view of the increased gain anddecreased noise and in further view of the fact that even the narrowerbandwidth of 500 megacycles represents a very broad band.

Noise ligure In the conventional traveling wave tube, `assuming noise tobe introduced only by fluctuating convection currents on the enteringelectron stream, the noise figure is given as r=1+2v2 2 T7" 0=1+s0v2v0cwhere 'y2 is the space charge noise reduction factor.

For the case in which the transit angle is very large or where there istemperature limitation, 72:1 `and we have pure shot noise. Then:

FT=1+80 Voc Substituting typical values, viz. Vo=1600 volts and C=.025,gives F=320l or about 35 decibel.

Actually, it is reasonable to suppose that 'y2 may be as small as 0.1 inwhich case the noise iigure calculates out to be about decibels and sometraveling wave tubes have been measured that Iare almost this good.

In the hybrid tube, under the assumptions of:

a. very short transit angles in the cathode-grid and gridhelix regions;

b. no partition noise before the stream reaches the helix;

and

c. high transconductance so that we may neglect the effect of anyvelocity modulation in the input space,

we may write the theoretical minimum noise ligure FH for the hybrid tubeas FFH-meng;

Tczcathode temperature in degrees Kelvin, and, T=temperature of inputtermination.

Typical values vare Tc: 1020, T=290,

giving 1:11220 or about 13 decibels or about one-half the number ofdecibels -to be expected from a good traveling wave tube.

Actually, this is essentially the same noise figure to be expected from`a close spaced triode under the same assumptions. Actual close spacedtriodes have been measured with noise figures between 14 and 18decibels. Any input passive loading would degrade the noise figure ofboth triode and hybrid alike so that 13 decibels actually represents aminimum figure.

For a detailed consideration of the equations relating to traveling wavetubes, reference may be had to a book entitled Traveling Wave Tubes byJ. R. Pierce, Van Nostrand, 1950. For a consideration of the equationsrelating to close spaced triodes, reference may be had to an articleentitled Design factors of the Bell Telephone Laboratories 1553 triodeby R. M. Ryder and applicant, appearing in the Bell System TechnicalJournal for October 1950 at page 496.

I ilustrativa embodiments The major components of a hybrid tubeembodying principles of the invention may be seen by referring to Fig.l. The tube there shown has an input section 11 `comprising the cathodeand grid sections of a microwave triode, a wave interaction pathcomprising the helix 12 through which an electron stream is directed andalong which a signal wave to be amplified is propagated, an outputcor-.pier 13, and a collector 14 for the electron stream. The inputsection 11 is enclosed in a metallic housing and the helix 12 issurrounded by a glass envelope and the entire structure is evacuated.Direct current coupling With which to apply bias voltages to thecollector 14 and helix 12 is provided by an inner metallic sleeve member15 connected to the collector 14 by means of a lead 16, and an outermetallic sleeve member 17 insulated from the inner member 16 by theglaze material 18 and connected to the helix 12 by an insulatedconductor 19 which may be seen by reference to Fig. 2A and the coupler13. A few loops 20 are provided in the lead 16 to serve both as a radiofrequency choke and as a spring to assist in holding the collector 14 inposition.

Means by which an input signal may be applied to and an output signaltaken from a tube of the type shown in Fig. 1 may be seen by referenceto Fig. 2A. An electron emissive cathode 31 is housed in cylinder 32 anda heating coil 33 is located within the hollow portion of the cathodecylinder. Biasing potentials from batteries 34 and 35 are applied to thecathode 31 and heater 33 by the socket pins 36 which extend through thebase of the cathode housing cylinder 32. An input cavity 37 is formed bythe top of the cathode housing cylinder 32 and a plate 38 having acentral aperture which houses the control grid 39. The circular sidewalls of the cavity 37 are composed of a vitreous material such as glasswhich is substantially transparent to microwaves. The cathode 31 andcontrol grid 39, together with a collimating grid 40 whose uniqueutility will be described below, form an electron gun assembly. Thehelix 12 and collector 14 are biased highly positive, for example, onthe order of 1500 volts, with respect to the cathode by battery 45, sothat an electron stream flows from the cathode 31 to the collectorcentrally along the longitudinal axis of the helix. Focusing of the beamis aided by a magnetic coil 46 housed in a cylindrical nonmagneticshield 47 which is concentric with the elongated portion of the tubewhich houses the helix 12. Direct current is supplied to the focusingcoil 46 by battery 48 and is adjustable by means of a potentiometer 49.

Microwave signals to be amplied from a source 50 are launched by meansof the antenna-like extension 51 of the central conductor of the coaxialcable 52 in the rectangular wave guide 53. The tube 11 extends throughthe guide 53 at a point substantially a quarter of a wavelength from thelower closed end with the tube axis normal to the broad faces of therectangular guide. A pair of annular anges S4 and 5S extend inward fromeither face of the guide flush with the outer surface of the cathodehousing cylinder 32 and with the circular edge of the apertured plate 38which mounts the control grid. At their farthermost extent into theguide 53, the ends of flanges 54 and 55 are flush with the top ofcylinder 3:2 and the left face of plate 33, respectively. Theelectromagnetic waves in the guide 53 induce standing waves in the inputcavity 37 which modulate the electron stream convection current passingtherethrough. As the stream enters the region within the helix, thevariations in the stream cause a radio frequency wave corresponding tothe input signal from the source 50 to be launched and impressed uponthe helix. The signal wave travels along the helix with a longitudinalphase velocity component substantially equal to the average electronvelocity, the pitch of the helix being proper, and interaction betweenthe stream and the longitudinal electric field components of the wavecauses the wave to grow in amplitude until it reaches the far end of thehelix 12. At this point, the antcnna-like extension 21 of coupler 13couples the amplied wave into an output rectangular wave guide 56 andthence to a load 57.

By way of comparing the transit angles and methods of signal input withconventional traveling wave tubes there are shown in Fig. 2B the majorcomponents of a traveling wave tube of the type disclosed in the bookand Y'arrastrar applications of J. R. Pierce mentioned above. Thesecomprise the electron gun assembly 56,. the input coupler- 57 havinganantenna-like extension 58 to which is welded the end of the helix 59 aninput wave guide-63, and helix support rods 41. With such a tube thetransit angle is of necessity suliiciently large so` that the beamenters the helix as substantially pure shot noise.

Referring now to Fig. 3, the input section 11 of the device of Fig. lutilizes techniques disclosed in Patent 2,502,530, dated April 4, 1950,of which I am a joint inventor together with R. L. Vance to effect asmall transit angle between the electron source and the helicaltransmission line. Therein is disclosed a space discharge devicecomprising a cathode, one or more grids, and an anode, capable ofoperating at ultra-high frequencies. For example, such a deviceemploying one grid, and known as a close spaced triode, has been foundat an operating frequency on the order of -4000 megacycles to have abandwidth of from 100 to 200 megacycles and a gain of approximately 9decibels. In this patent it is pointed out that extremely smallelectrode spacings, for example of the order of 1/2 `to l mil betweencathode and grid and of the order of to l0 mils between grid and anode,are achieved whereby electron transit times are minimized and hightransconductance and good signalto-noise ratio and band width areobtained. A close spaced triode is also described in the Bell SystemTechnifcal journal article mentioned above and one of such devices isknown commercially as the Western Electric 416A.

The device disclosed in the latter articles is characterized by inputand output sections of relatively low and high Qs respectively so thatthe bandwidth is limited chiefly in the output section. The presentinvention replaces the anode and output section of a close spaced triodewith a helical transmission line of a traveling wave tube. Helices haverelatively unlimited bandwidths at ultra-high frequencies so that t-hebandwidth of the combination or hybrid tube is that of the input sectionwhich in specific embodiments tested was found to be 1 on the order ofSGO megacycles.

The cathode-gridsection illustrated in Fig. 3 is similar to a` greatextent to the close spaced triode disclosed in Patents 2,455,381, datedDecember 7, 1948, to J. A. Morton and L. l. Speck, 2,502,531, datedApril 4, 1950, to LA. Morton and R. M. Ryder, and also 24,527,127, datedOctober 24, 1950, to R. S. Gormley, C. Maggs and L. F. Moose. Thedetailed description in these patents', as well as the techniques andmethods disclosed by them archereby incorporated the present disclosureand will be described but briefly herein by way of illustration.

The container of the inputsection comprises a cylindrical metallic shell61 provided with an outer lange 62 at one end and an inner lflange 63`at the other. A control grid terminal ring 64 is mounted on the flange63 byV a vitreous spacer 65 hermeticallys'ele'd between the parallelfacing surfaces ofthe ring 64 and the flange 63 of the shell. Aconductive path to the planar control grid 66 is by way of the circulargr'idconnector 6 7.

The cathode assembly is precision` fabricated to insure positiveparallelism with the wire's of the grid an'dmaintain close spacingthereto on the order of 1/2 mil. The cathode assembly comprises ametallic'sleeve` member 68 both to enclose the cathode heater' element69 and to support the rigid metallic 'discs 70V and `71. The disc 71provides a stable flat surface for 'the electron einis'siv'c coatingapplied thereto.

About the sleevel member 63 and 'coaxial therewith is an annular spacerring 72 of ceramic material. The spaer ring is maintained insubstantially coplanar relation with the @missive surface of the cathode71 by support members, not shown, which are attached about the peripheryof the sleeve member 68 `and extend to slots `provided :in thespacerring72.

The stem closure 4is a dished-metallic member 75 with apluralityofl'apertures inthe. base: portionA andi aperipheral flange 76 tocorrespond to the ange" 62 o'f the: shell 61.. The flanges 62 and 76 arering sealed to form a strong tightV joint between the Shelli and stem.Terminal pins 7'7 and 77' extend through the apertures inthe base memberand are supported therein by glass beads 78 which are hermeticallysealed to the pins and to the metallic eyelets 79. Two ofthe pins 77"support a heater shield 80 coaxially mounted about the shield member 68by means of the support members 81 joined to and insulated from the pins77 by the glass beads 82. These two pins 77f also provide aconductive'path for the heater element 69 to its supply 35, indicatedschematically in Fig. 2, by means of the conductor strips 83.

A low frequency coupling connection is provided from the other pin 77 tothe cathode by means of a conducting strip d5, a cylindrical' condensercan 86, and a cathode connector ring 87 which is in contact with thesleeve member 68. A high frequency coupling for the active cathodesurface is provided by they condenser can 86 and shell 61 to `anexternal circuit. In Fig. 2 the low frequency connection to the cathodeis' indicated by the lead 43 and the high frequency connection by themembers 44.

To insure accurate parallel relation between the Wires of the controlgrid 66 and the active surface of the cathode by a definite spacing onthe order of 1/2 mil, a spacer shim comp-rising a ring-like wafer 3S isinserted between the ceramic spacer ring '72 and the frame of thecontrol grid 66. (As previously explained, the ceramic ring 72 is in asubstantially coplanar relation with the emissive surface of the cathodeelement 71.)

Holding the cathode connector ring S7 in proper position and coaxialwith the spacer ring 72 is a second ceramic ring 91. A helical spring 92is seated in the metallic spring support 93 which is welded to the shell61 and the stern closure 7S. The `spring 'presses against the ring 9i,forcing all components towards the grid connector ring 67 and the dishedceramic ring 94 which mounts the collimating grid 95 and the helixsupport rods 96. All elements are thus held in accurate relation by thespring 92 during the operating life of the device.

The collimating grid 955, which will be described hereinafter in detailwith particular reference to Fig. 6 is spaced from and maintained inparallel relation with the control grid 66 by means of the inner flange97 of the control grid connector 67 `and the ceramic piece 94. The helix12 -is welded directly to the frame of the collimating grid 9S and Iismatched thereto by a distortion 98 of the helix. Referring to Figs. land `2, the helical transmission line 12 is matched to the output waveguide by means of the conductive coupler `ring 13 which has an antennaportion 21 welded to the helix. rlhe conical collector 14 for theelectrons of the electron beam is insulated from the coupler 13 by aceramic ring 22.

It has previously been mentioned that ion oscillations in plasma withinthe tube are a source of noise. Oscillations of this type are describedin detail in an article entitled Oscillations in ionized gases by Tonksand Langmuir in the Physical Review, volume 33, second series, 1929.Brieily, molecules of gas which remain in the tube after evacuation orwhich enter later due to an imperfect seal or which are subsequentlyliberated from the parts by electron bombardment and heating drift intothe region of the electron beam where they are ionized. These ions aretrapped in the depressed potential along the beam due to the electronspace charge. A beam subject to cross-over will have maximum currentdensity at a cross-over point. As shown by the cross-sectional potentialvdiagrams of Fig. 4A, the deepest potential pocket will therefore existat the cross-over points due to the more intense space `charge in theseregions. Hence the density of positive ions in the .region 'of theelectron beam will befgreatest `at the cross-overA points.

lf there are venough molecules inthe. tube, the'beam -Vwill eventuallybecome a plasma in which densities of ions and electrons are equal andthe total space charge will be zero. `As described in greater detail byTonks and Langmuir, such a medium is capable of supporting oscillationsof the ions due to random displacement. These oscillations have beenfound to appear as sidebands of the signal wave and as little as decibeldown from the signal. Such sidebands will not only distort the signalbut will also interfere with close space frequency multiplexing.

There are several Ways to prevent such oscillations. One way, largelytheoretical, is to obtain a perfect vacuum, or at least one yso hardthat the few molecules of gas remaining in the tube will not be able toionize and form a plasma capable of supporting ion oscillations. Anotheris to reduce the voltages and the length of the electron beam so thatthe ionization rate will be decreased. A further method, as disclosed inmy copending application Serial No. 136,206, filed December 31, 1949(United States Patent 2,692,351, issued October 19, 1954), is to sweepthe positive ions away from the region of the electron stream so thatplasma will not form.

Another method and a method with which the present invention isconcerned is to prevent the formation of deep potential pockets alongthe electron beam by using a parallel-sided beam. In the absence ofdensity modula* tion such a beam if perfectly collimated as shown linFig. 4B will have a substantially uniform electron density along itslength. The beam therefore Will not give rise to the potential pockets.

It is, of course, possible to launch a collimated beam by the use of acarefully designed electron gun. The present invention, however, isconcerned with the use of an auxiliary grid to collimate the beam.

With only a cathode 10i, control grid 102, and helix 103 as representeddiagrammatically in Fig. 5A and which, for example, are at 0, -l and+1700 volts potential respectively, the electric eld will have aconfiguration as illustrated by the dashed field lines. Due to the steeppotential gradient between the planar control grid and the cylindricalhelix, the field is distorted about the helix input, and a beam launchedthrough such a field configuration will be subject to cross-over.

If another planar grid idd is inserted between the helix and the controlgrid and maintained at approximately the same potential as the helix,the field will not be distorted but will appear as shown in Fig. 5B. Thesteep potential gradient is now between the coplanar grids where theelectric field line will be substantially straight, and, since the helix103 and the collimating grid 104 are at the same potential there will beno distorting field at the helix input so that the electrons emittedfrom `the cathode will tend to be propagated in a parallel-sided beam.

With reference now to Figs. 3 and 6, a grid 95 to collimate the electronlstream is interposed between the control grid 66 and the helix l2. Thegrid 95 comprises a tungsten mesh, lil held between two discs 112 ofconducting material. The conducting discs are welded together, thewelding being aided by a third disc 113 of suitabile material, and thehelix l2 is welded to the completed grid assembly as is more fullydisclosed in the first of my aforementioned applica-tions. The grid 95and helix 1.2 are thus maintained at the same potential so as to producethe field configuration of Fig. 5B and corn sequently a parallel-sidedbeam.

The completed grid assembly 9S fits about the support rods 96 by meansof the slots .1.14 provided in the grid frame and is seated in theceramic cup 94 which insulates the colflimating grid from the controlgrid connector 67.

It may be seen that the hybrid tube structure is especially adapted tothe use of a collimating grid and that its compact assembly is notdisturbed by the addition of such a grid, if desired. As has beenindicated, the collimating grid is not essential to the operation of thetube as an amplifier. Therefore, if the additional noise reduction to behad with use ofthe collimating grid is not necessary or desired, thatgrid may be omitted from the structure and the input section of Fig. 2Awould then be as illustrated in Fig. 2C. it may be noted that thecollimating grid di! of Fig. 2A does not appear in Fig. 2C so that theturns of the helix 12 in Fig. 2C are immediately adjacent to the controlgrid 39 rather than to an interposed collimating grid as shown in Fig.2A. Also, without av collimating grid the assembly of Figs. 3 and 6wil-l not be required.

Fig. illustrates a suggested modification of a hybrid tube when used asa modulator. The modulating signal is applied to the control grid 162from a source 105 as previously described, but the helix 12 is movedaway from the electron source so that the beating oscillator signal maybe launched down the helix as in the traveling wave tubes of the priorart (see Fig. 2B). Such an arrangement, of course, entails a sacrificeof signal-to-noise ratio due to the increased transit angle between thecathode 101 and helix l1.

The modulator suggested in Fig. 9 makes no sacrifice in signal-to-noiseratio, but, due 'to the added capacitance 166 in the input circuit, thebandwidth is decreased. The capacitance 106 isolates the localoscillator from the signal source 105 and is resonated at the signalfrequency by the inductance 167.

These and many other modifications will readily occur to one skilled inthe art and the fact that the invention has been described as relatingto a specific embodiment should not limit the appended claims.

Wherever in the description or claims the expression immediatelyadjacent is used it is to be understood to mean that two objects sodescribed have nothing comparable to either between (or separating)them. For instance, when two electrodes are immediately adjacent, thespace between them is electrode free.

What is claimed is:

l. A high frequency space discharge device comprising a cathode having aplane face, a planar grid member, means for spacing the plane face ofsaid cathode and said planar grid in aligned parallel manner, meansdefining an input cavity which includes the gap between said cathodeface and said grid member, a wave transmission path comprising anelongated helix having one end immediately adjacent said grid member andcoaxially aligned with said cathode and grid member and capable ofproducing an alternating electromagnetic field in an interaction spacesubstantially contiguous with said grid member for interacting with anelectron stream from said cathode, and an electron collector at the endof the helix remote from said cathode.

2. A high frequency electron discharge device cornprising a wavetransmission path, said path comprising an elongated helix, means topropagate a stream of electrons along a path adjacent and parallel tothe longitudinal axis of said helix comprising an electron emissivecathode at one end of said helix and an electron collector at the otherend of said helix, a first planar grid member interposed between andcoaxially aligned with said cathode and said helix, a second planar gridmember interposed between and aligned with said first grid member andsaid helix to collimate the electron stream entering the helix, saidstream characterized by a region of potential minimum resulting from theeffect of space charge in the vicinity of said cathode, and supportingmeans holding the end of said helix nearer said cathode immediatelyadjacent to said second grid member and as close as possible to saidpotential minimum.

3. An electron discharge device comprising a source of electrons havinga plane face, a first planar grid member, a second planar grid member,farther removed from said source than said first grid member, meansspacing said face of the source and said planar grid members in alignedparallel manner, `means definining an elongated wave transmissioninteraction path having one end located immediately adjacent to saidsecond grid member whereby the electrons from said source are collimatedimmediately before reaching the wave interaction path and the electrontransit angle from said source to said path end is substantially lessthan the minimum transit angle necessary for random fluctuations in anelectron stream proceeding from said source to said wave interactionpath to reach pure shot noise, and a collector of electrons located atthe end of said wave interaction path remote from said source.

4. A high frequency space discharge device having an input sectioncomprising a cathode member having a planar emissive face, a firstplanar grid member adjacent the face of said cathode, a. second planargrid member adjacent said grst grid member, means to insulate saidsecond grid member from said first grid member, means spacing saidcathode face and said grid members in aligned parallel relation, andmeans defining an input cavity which includes the gap between saidcathode face and said first grid, an electrical wave transmission pathcomprising an elongated helix having one end immediately adjacent to andconnected to said second grid member, means coaxially aligning saidcathode, said grid members and said helix and an electron collector atthe end of said helix remote from said cathode.

5. The combination in accordance with claim 4 and a plurality of supportrods longitudinally disposed about the outer surface of said helix andseated at one end in said second grid member.

6. An electron discharge device comprising a source of electrons havinga plane face, a rst planar grid member adjacent said face, a secondplanar grid member adjacent said rst grid member, means to insulate saidsecond grid member from said first grid member, an elongated wavetransmission path connected at one end to said second grid member, acollector of electrons located at the other end of said path, meansspacing said face and said grids in aligned parallel relation, saidtransmission path end being located immediately adjacent to said secondgrid member whereby the electron transit angle between said source andsaid transmission path end is substantially less than the transit anglerequired for convection current liuctuations in the electron streamemanating from said source to reach the level of pure shot noise, andmeans to modulate said stream before it reaches said wave transmissionpath.

7. The combination in accordance with claim 6 Wherein said last-namedmeans comprise means defining a cavity resonator which includes at leasta portion of the gap between said source and said first grid member, andmeans responsive to a signal to be amplified to induce standing waves insaid cavity.

8. A high frequency space discharge device comprising a helicaltransmission line, means to cause a stream of electrons to be propagatedalong a path parallel and adjacent to the longitudinal axis of saidhelical line, said means comprising a cathode, a control grid, and acollimating grid, said collimating grid positioned immediately adjacentto one end of said helical line and transverse to said axis, saidhelical line connected directly to said collimating grid, and meanscomprising an input cavity resonator which includes the gap between saidcathode and said control grid to modulate said stream.

9. A space discharge device for amplifying high frequency signal wavescomprising a transmission line at least several wavelengths long, saidline comprising a helix of uniform pitch along the greater portion ofits length, means to cause a stream of electrons to be propagated alonga path adjacent and parallel to the longitudinal axis of said helix,said means comprising a source of electrons, a control grid adjacentthereto, and a collimatng grid interposed between said control grid andone end of said helix immediately adjacent to said end of helix, saidcontrol grid and said collimating grid in aligned parallel relation andcoaxial with said helix, and means to modulate said stream as ittraverses the space between said source and said control grid.

l0. An electron discharge device comprising a first support memberhaving a plane seating surface, a cathode mounted by said support memberand having a face coplanar with said surface, a second support memberhaving a plane seating surface, a first planar grid member, a secondplanar grid member seated in said second support member, means spacingsaid cathode face, said first grid member and said second grid memberfrom each other in aligned parallel relation comprising spacer meansbetween said grid member and the seating surfaces of said first and saidsecond support members, means separate from said first and secondsupport members locking said support members against said spacer means,a transmission line at least several wavelengths long immediatelyadjacent to and connected directly to said second planar grid, said linecomprising a helix which has a substantially uniform pitch along agreater portion of its length, support means to mount said helixcoaxially with said cathode and said grids, said support meanscomprising a plurality of rods of insulating material spacedsymmetrically about said helix in a longitudinal manner and held in firmcontact therewith, and electron collector means adjacent to the end ofsaid helix remote from said second grid.

11. An electron discharge device comprising a helical transmission lineat least several wavelengths long rigidly supported by a plurality ofsupport rods spaced about said helix in a longitudinal manner, means tocause a stream of electrons to be propagated along a path adjacent andparallel to the longitudinal axis of said helix, said means comprising acathode, a first plane grid, and a second plane grid each axiallydisplaced from said helix, with said second grid immediately adjacent tosaid helix, said cathode having a plane face, said face and said firstand second grids in aligned parallel relation and means to space saidcathode and said grids from each other in accurate relative positions,and electron collector means at the other end of said helix from saidcathode and said grids.

l2. A high frequency space discharge device comprising a cathode havinga plane face, a planar grid member, means for spacing the plane face ofsaid cathode and said planar grid in aligned parallel manner, meansdefining an input cavity which includes the gap between said cathodeface and said grid member, said cavity being the sole means forimpressing signal wave energy from an external signal source on anelectron beam from said cathode, an elongated helix coaxially alignedwith said cathode and grid member and having one end adjacent the` gridmember for defining a beam interaction space whlch 1s substantiallycontiguous with said grid member,

and an electron collector at the end of the helix remote from saidcathode.

References Cited inthe file of this patent UNITED STATES PATENTS2,064,469 Haeff Dec. 15, 1936 2,300,052 Lindenblad Oct. 27, 19422,502,530 Morton et al Apr. 4, 1950 2,521,760 Starr Sept. 12, 19502,575,383 Field Nov. 20, 1951 2,595,698 Peter May 6, 1952 FOREIGNPATENTS 934,220 France Ian. 7, 1948 OTHER REFERENCES Article by Kompfnerin Proceedin s of I. R. E. 124421. February 1947. g PP

